The three-hour Pollutant Standards Index (PSI) on Sunday afternoon dipped to its lowest since the haze blanketed Singapore two weeks ago. The reading at 3pm stood at 17, while the 4pm reading was 18. Both are in the "good" air quality range. The 24-hour average PSI at 4pm was 57-62, down from 58-63 the day before. Air in the 51-100 PSI range is considered "moderate". A reading of between 101 and 200 is considered "unhealthy", between 201 and 300 is "very unhealthy", and anything above 300 is hazardous. However, the 24-hour average readings of PM2.5 concentration - referring to particles which are 2.5 microns or smaller - remain high at 46-61. Health advisories kick in when that figure breaches 40. The rains that swept through most parts of Singapore early Sunday could have contributed to the decreasing PSI readings, but studies have shown that rain does not fully wash the ...